A double-stranded DNA probe synthesized from the EIAV genome revealed 146 to 228 probe equivalents integrated into the genome of infected equine fibroblasts in vitro. Infected macrophages contained almost 500 probe equivalents per cell. The EIA virion is composed of 5 major and 9 minor proteins that generally resemble the polypeptide patterns of other retroviruses. Peptide maps suggest that gp 77/79, gp 64 and gp 40 originate from a common precursor molecule. Persistent infection by EIAV causes no measurable effect on cell physiology nor any sign of malignant transformation. Cell DNA synthesis is not required for successful EIAV infection. Synthesis of viral protein fluctuates, depending on the stage of the cell cycle, whereas virion output is steady, even in long-term stationary cultures. Fibroblasts are about 100-fold less sensitive to EIAV infection than are equine macrophages. Infected horses develop sensitive lymphocytes that are capable of killing EIAV-infected cells by a direct cytotoxicity mechanism. Horses are also capable of an efficient antibody-dependent cellular cytotoxicity reaction, but this mechanism was not found operable against EIAV-infected cells.